19 CFR §113.25

When a seal is required, the seal must be affixed adjoining the signatures of principal and surety, if individuals, and the corporate seal must be affixed close to the signatures of persons signing on behalf of a corporation. Bonds must be under seal in accordance with the law of the state in which executed. However, when the charter or governing statute of a corporation requires its acts to be evidenced by its corporate seal, such seal is required.